1. Field of the Invention
This invention relates to a heat-sensitive recording sheet which is superior in the heat resistance of the background, and the water resistance, oil resistance and heat resistance of image.
2. Prior Art
In general, a heat-sensitive recording sheet is produced by applying on a support, such as paper, synthetic paper, film, plastic, etc., a coating material which is prepared by individually grinding and dispersing a colorless chromogenic dye and an organic color-developing agent, such as phenolic material, etc., into fine particles, mixing the resultant dispersions with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. The coating, when heated by thermal pen, thermal head, hot stamp, laser beam, etc., undergoes instantaneously a chemical reaction which forms a color. These heat-sensitive recording sheets have now been finding a wide range of applications, including industrial measurement recording instruments, terminal printers of computer, facsimile equipment, automatic ticket vending machines, printer for bar-code-label, and so on. In recent years, as the application of such recording is diversified and the performance of such recording equipment is enhanced, high qualities are required for heat-sensitive recording sheets. For example, even with small heat energy in a high speed recording, both the clear image with a high density and the better preservability such as better resistance to light, weather and oil, etc. are required. The conventional heat-sensitive recording sheets are disclosed, for examples, in the Japanese Patent Publication Nos. 43-4160 and 45-14039.
However, these heat-sensitive recording sheets have as a defect, for example, an insufficient image density in high speed recording owing to the inferior thermal responsibility.
As the methods for improving the above defect, there have been developed the high sensitive dyes, for example, 3-N-methyl-N-cyclohexylamio-6-methyl-7-anilinofluorane (in the Japanese Patent Laid-Open No. 49-109120) and 3dibutylamino-6-methyl-7-anilinofluorane (in the Japanese Patent Laid Open No. 59-190891) as leuco dye. Further, there have been proposed the color-developing agents of superior color-formation, for example, 1,7-bis(4-hydroxyphenylthio)-3,5-dioxaheptane (in the Japanese Patent Laid-Open 59-106456), 1,5-bis(4-hydroxyphenylthio)-3-oxaheptane (in the Japanese Patent Laid-Open No. 59-116262) and 4-hydroxy-4'-isopropoxydiphenylsulfone (in the Japanese Patent Publication No. 63-46067). Using these chemicals, the technologies for the recording with high speed and sensitivity have been disclosed.
Although these heat-sensitive recording sheets provide a high sensitivity, they have a problem in an inferior heat-resistance, i.e. a formation of background fogging in the high temperature storage.
Further, they exhibit the extremely inferior preservability of the recorded image, which causes the following defects. In the adhesion to moisture or serum, or in the contact with plasticizer (DOP, DOA, etc.) in a wrapping film, the image density is prominently decreased or the recorded image disappears.
Further, Japanese Patent Application No. 1-267590 discloses a technique providing a heat-sensitive recording sheet which is superior in heat resistance, water resistance and oil resistance. In this case, however, there are problems in that a heat-recording sheet is inferior in preservability(heat-, water-, and oil-resistance), somewhat insufficient dynamic image density and causes slight formation of background fogging.